U.S. patent application number 09/764028 was filed with the patent office on 2001-07-19 for bandpass-limiting device for a receiver.
This patent application is currently assigned to VERTEX STANDARD CO., LTD.. Invention is credited to Hashimoto, Yoshiteru.
Application Number | 20010008549 09/764028 |
Document ID | / |
Family ID | 18536604 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008549 |
Kind Code |
A1 |
Hashimoto, Yoshiteru |
July 19, 2001 |
Bandpass-limiting device for a receiver
Abstract
In a bandwidth-limiting apparatus, control is performed of the
oscillation frequencies of each of three local oscillators of three
intermediate-frequency circuits, and the passband is limited by
adjusting the width of and shifting the IF, the intermediate
frequency at the last stage being converted by a frequency
conversion means to a low frequency suitable for processing, which
is then bandwidth limited by a filter function of a digital signal
processor. Control of each passband is performed in linked fashion
by control stored in a microcomputer circuit, and the sharp shape
factor of the digital signal processor is effectively used to
eliminate noise from the deviation passband. Because noise is
eliminated before the detection stage, there is no cause for the
intrusion of wraparound noise, thereby eliminating the necessity to
perform excessive bandwidth limiting.
Inventors: |
Hashimoto, Yoshiteru;
(Tokyo, JP) |
Correspondence
Address: |
Gary M. Anderson
Fulwider Patton Lee & Utecht, LLP
200 Oceangate, Suite 1550
Long Beach
CA
90802
US
|
Assignee: |
VERTEX STANDARD CO., LTD.
|
Family ID: |
18536604 |
Appl. No.: |
09/764028 |
Filed: |
January 16, 2001 |
Current U.S.
Class: |
375/316 ;
375/350 |
Current CPC
Class: |
H04B 1/0003 20130101;
H04B 1/1036 20130101; H04B 1/14 20130101 |
Class at
Publication: |
375/316 ;
375/350 |
International
Class: |
H03K 009/00; H04L
027/06; H04L 027/14 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 17, 2000 |
JP |
2000-008312 |
Claims
What is claimed is:
1. A bandpass limiting apparatus in a receiver of the
superheterodyne type, in which a plurality of frequency conversions
are performed, said apparatus comprising: a bandpass changing means
for controlling an intermediate-frequency circuit and broadening,
narrowing, and/or shifting a passband of an intermediate-frequency
signal obtained from a final stage thereof; an A/ID conversion
means for converting said intermediate-frequency signal to a
digital signal; a digital signal processing means for broadening,
narrowing, and/or shifting the passband of said
intermediate-frequency signal converted to a digital signal by said
A/D conversion means; a detection means for obtaining an audio
signal detected from said digital signal output by said digital
signal processing means; a passband changing means changing a
passband based on an adjustment signal from an adjustment operation
part; and a control means, which, by controlling said digital
signal processing means, based on an adjustment signal from an
adjustment operation part, causes said passband of said
intermediate-frequency signals at each stage to change in
concert.
2. A bandpass limiting apparatus in a receiver of the
superheterodyne type, in which a plurality of frequency conversions
are performed, said apparatus comprising: a bandpass changing means
for controlling an intermediate-frequency circuit and broadening,
narrowing, and/or shifting a passband of an intermediate-frequency
signal obtained from a final stage thereof; a frequency conversion
means for changing said intermediate-frequency signal to a
frequency signal for processing that is suitable for a data
processing speed of a digital signal processing means; an A/D
conversion means for converting said intermediate-frequency signal
to a digital signal; a digital signal processing means for
broadening, narrowing, and/or shifting the passband of said
intermediate-frequency signal converted to a digital signal by said
A/D conversion means; a detection means for obtaining an audio
signal detected from said digital signal output by said digital
signal processing means; and a control means, which, by controlling
said digital signal processing means, based on an adjustment signal
from an adjustment operation part, causes said passband of said
intermediate-frequency signals at each stage to change in
concert.
3. A bandpass-limiting apparatus according to claim 1 or claim 2,
wherein said detection means converts a digital signal obtained
from said digital signal processing means to a digital signal
corresponding to an audio signal after detection, the converted
digital signal being then D/A converted.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a bandpass-limiting device
for a receiver, suitable for use in an SSB radio receiver or the
like, and more particularly to a bandpass-limiting method using a
digital filter for the purpose of effectively eliminating unwanted
signals.
RELATED ART
[0002] Recently, digital signal processors (DSPs) have been used in
the field of radio communications for the purpose of signal
processing, and transceivers exist which utilize low-frequency
filters or intermediate-frequency filters implemented using
DSPs.
[0003] A filter formed by a DSP is free from variations in various
characteristics, such as experienced with analog filters (active
filters implemented with elements such as operational amplifiers),
and do not require consideration of such factors as temperature
variation and aging. In particular, a digital filter can be
precisely set to desired amplitude characteristics and shape
factor, exhibiting such desired characteristics according to
theory, and has the advantage of being able to achieve a sharp
shape factor that is close to the value 1.
[0004] In a superheterodyne radio receiver in which a plurality of
frequency conversions are performed, an intermediate frequency (IF)
bandpass filter (BPF) can be implemented as an analog filter, and a
DSP filter can be applied as a low-frequency filter after
detection, the local oscillation frequencies of local oscillators
used for the purpose of converting frequency in each frequency
mixer to an intermediate frequency being controlled, and a means
for changing the bandpass (IF width or IF shift) is combined with a
low-frequency DSP filter, variable control being performed of each
filter independently, so as to achieve a desired passband at a
received frequency, resulting in an audio output from which
unwanted signal interference and noise have been removed.
[0005] However, the adjustment of the IF width and IF shift to
achieve a desired passband is established by an
intermediate-frequency filter, which is implemented as an analog
filter, and the DSP filter is often set so as to have a sharp shape
factor as noted above, it being difficult to perform variable
control of each filter while achieving coincidence between the
passbands of both, leading to the problem of not being able to
eliminate an unwanted signal with good efficiency.
[0006] Given the above, the inventor has proposed (in Japanese
Patent No. 2887789) a bandwidth-limiting system having, in a
multiple superheterodyne radio receiver with two or more frequency
conversions, means for adjusting the passband of an intermediate
frequency and a filter means for varying the passband for low
frequencies, using a digital signal processing means, these means
being linked and variable, and the passbands of each being
simultaneously and equivalently varied, so that, with respect to a
desired signal received by the radio receiver, the influence of an
unwanted signal that can occur is caused to be reduced, and also so
that this series of operations is entirely performed by a main
control means.
[0007] More specifically, this is an SSB (radio transmission modes
J3E and R3E) radio receiver and bandpass-limiting system, such as
shown in the system block diagram of FIG. 5.
[0008] A received signal (in the 50-MHz band) from the antenna 51
is amplified by an RF amplifier 52 and input to a first frequency
mixer 53, at which it is mixed with a local oscillator signal (60.7
MHz) from a first local oscillator 53a, after which a first
intermediate-frequency (IF) signal (10.7 MHz) is obtained, using
the first IF filter (BPF) 54. The above-noted first IF signal is
input to a second frequency mixer 55, at which it is mixed with a
local oscillator signal (10.7245 MHz) from a second local
oscillator 55a, so as to obtain a second IF signals (455 kHz),
using a second intermediate-frequency filter (BPF) 56.
[0009] The second IF signal is input to a detector 57, at which it
is mixed with a carrier signal (456.5 kHz) from a beat frequency
oscillator (BFO) 57a, thereby resulting in a demodulated SSB audio
signal (0.3 kHz to 2.7 kHz).
[0010] The above-noted audio signal is passed through an A/D
converter 58, the output of which is input to a low-frequency DSP
filter 59, at which bandpass limiting is perform, so as to removed
an unwanted signal, the resulting output from which being passed
through a D/A converter 60, which converts the signal to an analog
signal, which is amplified by an audio amplifier 61, and output as
an audio signal from a speaker 62.
[0011] In this radio receiver, each of the local oscillators 53a,
and 55a, and the DSP filter 59 are controlled in linked fashion by
a microcomputer circuit 71, this microcomputer circuit 71, based on
an adjustment signal of a bandpass adjuster 72, causing the local
oscillator frequency of the local oscillators 53a and 55a to change
by a minute step, so as to control the bandwidth by changing the IF
width and IF shift, the result being that the DSP filter 59 is
bandwidth limited by an equivalent amount to the above-noted limit
amount.
[0012] That is, the IF width and IF shift according to control of
the local oscillators 53a and 55a is simply bandwidth limiting of a
virtual filter FL (if) comprising an IF stage, a ROM 71b of the
microcomputer circuit 71 having stored in it a bandwidth limiting
amount according to this virtual filter FL (if) and a bandwidth
limiting amount of the DSP filter 59, this being stored as control
data for the purpose of achieving an equivalent change for linked
to these amounts, the CPU 71a executing control in accordance with
an adjustment signal of the bandwidth adjuster 72 obtained via the
interface circuit 71c.
[0013] However, the passband of the IF stage virtual filter FL (if)
is established by the passband limited by the bandpass limiting
established by the first IF filter 54 and the second IF filter 56
which are bandpass filters, and because these filters are analog
filters, it is difficult to achieve ideal characteristics having a
shape factor 1, and also impossible to achieve an attenuation
amount of "infinity" outside of the passband.
[0014] Therefore, it is impossible to avoid the intrusion of an
unwanted signal in the audio signal after detection, resulting in a
worsening of the S/N ratio, and if an unwanted signal is at the
carrier frequency, there is also a loss of clarity.
[0015] In this radio receiver, by providing a DSP filter 59 after
detection, the shape factor can be set to a value near 1, and
further the associated passband can be made to equivalently vary,
linked to the passband of the virtual filter FL (if), thereby
enabling accurate bandwidth limiting with simple operation, and the
achievement of an audio output with a high S/N ratio.
[0016] In the radio receiver shown in FIG. 5, the DSP filter 59
performs bandwidth limiting with respect to the audio signal
demodulated after the detection stage.
[0017] In detection of a received SSB (J3E or R3E) signal, the
carrier signal for demodulation from the BFO 57a as noted above
must be mixed in the detector 57 to obtain the demodulated signal.
However, the second IF signal before detection is subjected to
bandwidth limiting by the first IF filter 54 and the second IF
filter 56, so that, as shown in FIG. 6, because the shape factor of
the IF stage virtual filter FL (if) is not that great, the carrier
frequency for demodulation is within the deviation bandwidth.
[0018] In the above case, the frequency characteristics of the
virtual filter FL (if) wrap around into the passband at the carrier
signal frequency for demodulation, so that signal superimposition
causes wraparound noise to intrude, and in particular if there is
interference within the above-noted deviation bandwidth, this noise
is wrapped around and enters the passband of the DSP filter 59,
resulting in highly noticeable noise in the played back sound.
[0019] Therefore, even in the above-noted radio receiver in which
an equivalent change is made by linking the passbands of the
virtual filter FL (IF) and the DSP filter 59, it is not possible to
eliminate this wraparound noise, and if elimination is done, it is
necessary to make the passband of the DSP filter 59 even
narrower.
[0020] Accordingly, it is an object of the present invention to
provide a bandpass limiting apparatus that solves the above-noted
problem, making effective use of the characteristics of a DSP
filter that enables setting of a sharp shape factor.
SUMMARY OF THE INVENTION
[0021] The present invention is a bandpass limiting apparatus in a
receiver of the superheterodyne type, in which a plurality of
frequency conversions are performed, this apparatus having a
bandpass changing means for controlling an intermediate-frequency
circuit and broadening, narrowing, and/or shifting the passband of
the intermediate-frequency signal obtained from the final stage
thereof, an A/D conversion means for converting this
intermediate-frequency signal to a digital signal, a digital signal
processing means for broadening, narrowing, and/or shifting the
passband of the intermediate-frequency signal converted to a
digital signal by the A/D conversion means, a detection means for
obtaining an audio signal detected from the digital signal output
by the digital signal processing means, a passband changing means
that changes the passband based on an adjustment signal from an
adjustment operation part, and a control means, which, by
controlling the digital signal processing means, causes the
passband of the intermediate-frequency signal at each stage to
change in concert.
[0022] According the present invention, it is possible, using a
digital signal processing means (DSP) at the first stage of a
detection means, to cause broadening or narrowing and/or movement
of the passband, thereby enabling bandpass limitation with a sharp
shape factor.
[0023] Therefore, because the cause of wraparound noise in the
detection means is eliminated, it is possible to perform audio
playback with a high S/N ratio.
[0024] Although, in the present invention, the digital signal
processing means has a data processing speed that enables proper
processing of an intermediate-frequency signal, such DSP's having
this high processing speed are only used in specialized fields, and
are not only difficult to obtain but also high in cost.
[0025] Given the above, it is possible to provide a frequency
conversion means which converts the intermediate-frequency signal
to a frequency (frequency for processing) that is suitable for the
data processing speed of the digital signal processing means,
thereby enabling implementation with a conventional DSP.
[0026] In this case, the bandwidth of the frequency for processing
is broadened, narrowed, and/or shifted by the digital signal
processing means, and the control means causes a change in the
bandpass of the intermediate-frequency signal and the signal for
processing in concert, thereby achieving the same effect as the
present invention.
[0027] Although it is possible as a detection means to D/A convert
the digital signal obtained from the digital signal processing
means and to perform conventional analog detection, it is
alternatively possible have a configuration which converts the
digital signal to a digital signal corresponding to the audio
signal after detection, this converted digital signal being then
D/A converted.
[0028] In particular, development is being pursued of DSP's which
are capable of not only a filtering function but also a detection
function, so that they can perform detection at the digital signal
stage, thereby enabling a simplification of the circuit
configuration.
[0029] Thus, in a specific circuit configuration, the digital
signal processing means and detection means can be mounted on one
and the same chip or one and the same board, and it is further
possible to mount thereon an A/D conversion means and a D/A
conversion means.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The above and other features of the present invention will
be better understood by reading the description of exemplary
embodiments to follow, making reference to the accompanying
drawings, of which:
[0031] FIG. 1 is a system block diagram of a radio receiver
according to an embodiment of the present invention;
[0032] FIG. 2 is a drawing showing the frequency characteristics of
the intermediate circuit and the DSP filter function for the
purpose of illustrating of the principle of the present
invention;
[0033] FIG. 3 is a drawing showing frequency characteristics,
wherein bandpass control with respect to the intermediate frequency
circuit in the initial adjustment stage and bandpass control of the
DSP filter function are varied equivalently, after which shape
factor control is performed by means of the filter function of the
DSP;
[0034] FIG. 4 is a circuit block diagram for the case of the
configuration of an analog detection circuit;
[0035] FIG. 5 is a system block diagram of a radio receiver in the
prior art; and
[0036] FIG. 6 shows the frequency characteristics of an
intermediate-frequency circuit and a DSP filter for the purpose of
illustrating the problems in the prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] A preferred embodiment of a bandpass limiting apparatus for
a receiver according to the present invention is described in
detail below, with references made to FIG. 1 to FIG. 4.
[0038] FIG. 1 is a system block diagram of a radio receiver for SSB
(radio signal type; J3E or R3E) according to an embodiment of the
present invention.
[0039] In this radio receiver, first a received signal (in this
case, in the 10-MHz band) from the antenna 1 is amplified by an RF
amplifier 2 and input to a first frequency mixer 3, at which it is
mixed with a local oscillation signal (80 MHz) from a first local
oscillator 3a, thereby resulting in a first intermediate-frequency
signal (70 MHz) using a first intermediate-frequency filter (BPF)
4.
[0040] Next, the first intermediate-frequency signal is input to a
second frequency mixer 5, at which it is mixed with a local
oscillator signal (59.3 MHz) from a second local oscillator 5a, so
as to obtain a second intermediate-frequency signal (10.7 MHz)
using a second intermediate-frequency filter (BPF) 6.
[0041] Additionally, the second intermediate-frequency signal is
input to a third frequency mixer 7, at which it is mixed with a
local oscillator signal (10.245 MHz) from a third local oscillator
7a, so as to obtain a third intermediate-frequency signal (455 kHz)
using the third intermediate-frequency filter (BPF) 8.
[0042] Then, the third intermediate-frequency signal is input to a
fourth frequency mixer 9, at which it is mixed with a local
oscillator signal (444.76 kHz) from a fourth local oscillator 9a,
thereby converting it to a fourth frequency signal at 10.24
kHz.
[0043] This is done because it is not possible to apply the data
processing speed of DSP 11 to a signal at 455 kHz. However, if it
is possible to capture and process the third intermediate-frequency
signal (455 kHz) using the DSP 11, it is possible to eliminate the
fourth frequency mixer 9 and the fourth local oscillator 9a.
[0044] The converted fourth frequency signal has (10.24 kHz) is
converted to a digital signal by the A/D converter 10 and then
input to the DSP 11, which has a digital filter function (11a),
digital detection being done of a signal bandlimited by the
passband thereof using a detection function (11b), so as to obtain
a demodulated signal (digital signal). In the DSP 11, the
carrier-frequency signal for demodulation is generated internally
and a detection function is executed.
[0045] The audio signal that is demodulated from the received
signal is therefore output from the DSP 11 as a digital signal,
this being converted to an analog signal by the D/A converter 12,
after which this signal is amplified by the audio amplifier 13, and
output as a played back audio signal from the speaker 14.
[0046] In this radio receiver, similar to the case of the radio
receiver shown in FIG. 5, the microcomputer circuit 21, based on an
adjustment signal from the adjustment operation part 22 adjusts the
various local oscillation frequencies of the first local oscillator
3a, the second local oscillator 5a, and the third local oscillator
7a, and sends control data to the DSP 11, enabling variation of the
bandwidth characteristics of the associated digital filter.
[0047] That is, in addition to performing a control of the IF width
an IF shift, in the intermediate-frequency circuitry, the filter
function (11a) of the DSP 11 is also configured to perform
bandwidth limiting, the amount of limiting of these bandwidths
being set by a control program that causes storage beforehand into
the ROM 21b of the microcomputer circuit 21 and a control table.
The CPU 21a, in response to an adjustment signal from the
adjustment operation part 22, performs variation of the bandwidth,
by transferring control table data from the interface 21c to the
various local oscillators 3a, 5a, and 7a.
[0048] Taking the example of the intermediate-frequency circuitry,
in the case of varying the frequency of the first local oscillator
3a from 18.001 to 18.0015 MHz, varying the frequency of the second
local oscillator 5a from 59.299 to 59.300 MHz, and varying the
frequency of the third local oscillator 7a from 10.2450 to 10.2455
MHz, it is possible to vary the IF width over a maximum bandwidth
of 3 kHz and a minimum bandwidth of 2 kHz with the third IF
frequency of 455 kHz as a center frequency, and further possible to
vary the IF shift a maximum of 500 kHz.
[0049] The above-noted IF width and IF shift control is perform
using the cutoff characteristics of the IF filters 4, 6, and 8,
which are analog filters provided in each stage of the IF
circuitry.
[0050] Therefore, as shown in FIG. 2, the shape factor of the
virtual filter FL (if) of the overall IF circuitry becomes
relatively large, so that the deviation passband of the frequency
characteristics thereof widens, thereby increasing the possibility
of the intrusion of noise such as interference into the deviation
passband.
[0051] In the prior art, as shown in FIG. 5, after the detection
circuit 57, a DSP filter 59 is provided. However, as shown in FIG.
6, when detecting the IF signal, if the carrier frequency for
demodulation is within the deviation passband, there is intrusion
of wraparound noise, and it is impossible to eliminate this using
the DSP filter 59.
[0052] However, with a radio receiver according to this embodiment
of the present invention, even if noise such as interference enters
the deviation passband of the virtual filter FL (if) of the overall
IF circuitry, in the DSP filter function (11a), bandwidth limiting
is already perform using a filter characteristic having a shape
factor that is nearly 1, after which demodulation is performed by
the detection function (11b), so that this noise does not enter the
passband.
[0053] In this radio receiver, there is a linking between the
bandpass limiting amount by IF width and IF shift control and the
bandpass limiting amount by the DSP filter function (11a).
[0054] In this case, it is possible, such as in a radio receiver of
the prior art, to vary the various bandwidth limiting amounts by
equivalent amounts, in response to a control adjustment signal from
the adjustment operation part 22, although it is alternatively
possible to adopt a control method whereby the bandwidth limiting
amount of the IF circuit 3 is set so that it is larger than the
bandwidth limiting amount of the DSP filter functions (11a), and
bandwidth limiting by IF width and IF shift control is applied with
priority, as the bandwidth limiting by the DSP filter function
(11a), is gradually applied.
[0055] In addition, because the bandwidth control of the DSP 11
filter function (11a) and control of the shift factor thereof are
performed independently without mutual correlation, as shown in
FIG. 3, in the initial adjustment stage, the bandwidth limiting
amount by IF width and IF shift control and bandwidth limiting
amount by the DSP filter functions (11a) are made equivalent
(<1>) and at a stage at which the bandwidth has been somewhat
limited so that noise is assumed to attenuated, control is
performed so as to bring the shape factor only of the DSP filter
function (11a) close to the value 1(<2>), thereby enabling
the removal of noise components within the deviation passband to
outside the passband, without performing excessive bandwidth
control, facilitating perception of the adjustment condition as an
additional advantage.
[0056] In either case, as shown in FIG. 2, noise that enters
deviation passbands of the virtual filter FL (if) of the overall IF
circuitry is eliminated by the filter characteristics of the DSP
filter function (11a) which have a sharp shape factor, after which
the signal is subject to detection processing, so that there is no
possibility of a problem occurring with wraparound noise at the
detection stage, thereby making it possible to play back an audio
signal with a high S/N ratio.
[0057] In the above-noted embodiment, the DSP 11 has both a filter
function (11a) and a detection function (11b), this functions being
executed by means of digital signal processing. However, it will be
understood that it is alternatively possible, as shown in FIG. 4,
to impart only a filter function (11a) to the DSP 11, the output
signal from which is converted to analog form by the D/A converter
12, and demodulated to a audio signal by the analog detection
circuit 15, thereby providing an audio signal for playback.
[0058] In this case as well, because detection is performed after
bandwidth limiting done by the filter function (11a) of the DSP 11,
even if the frequency of the carrier for demodulation from the BFO
16 falls within the passband of the virtual filter FL (if) of the
overall IF circuitry, it is possible to avoid the introduction of
noise such as interference into the passband.
[0059] By adopting the configuration described in detailed above, a
bandpass limiting apparatus for a radio receiver according to the
present invention achieves a number of effects.
[0060] According to the present invention as recited in claim 1 of
the accompanying claims, a digital signal processing means is
provided before a detection means, and bandwidth limiting by a
digital signal processing means is done linked to IF circuitry
bandwidth limiting performed by means of IF width and IF shift
control. The result is that, even if noise enters the deviation
passband of the virtual filter characteristics of the circuitry,
this noise is eliminated by the sharp shape factor of the digital
signal processing means, after which detection is performed,
thereby solving the problem of noise wraparound of noise such as
interference into the passband, as was a problem in the past, and
enabling playback of received audio signal with a high S/N
ratio.
[0061] Another effect of the present invention is that, because
noise entering the deviation passband such as in the past is
eliminated, there is no need to perform excessive band pass
limiting.
[0062] According to the present invention as recited in claim 2 of
the accompanying claims, it is possible to use a conventional DSP,
as opposed to a DSP with a fast data processing speed, as the
digital signal processing means, thereby enabling a reduction in
cost of the radio receiver.
[0063] According to the present invention as recited in claim 3 of
the accompanying claims, the detection function is executed by
digital signal processing and both the filter function and the
detection function are incorporated into the DSP, thereby
simplifying the circuit configuration.
* * * * *